Good DOF and I like the composition. The lighting is harsh making the highlights a bit blown out, causing a loss of color and definition. Also, while this is a beautiful shot, I'm not sure I'd consider it a macro because I know that these particular shells get to be very large. Still a nice capture and I have to give the benefit of the doubt on the macro thing. 6
7 - This is nice, not sure how you got the sand grey or if it is natural. For me, a tad closer and finer focus on the foremost 'extensions' would have made this much better. I like the simplicity concept.
Lighting is very garish! Other than that, its a nice object. But a nice object isn't the same as a nice photo. There's little here to hold the eye or lead it to points of interest.
I find the shadow to be more distracting then anything. I also find the light source to be a little to bright. Perhaps playing with your brightness and contrast levels would have corrected this a little bit. Good luck in this challenge. <6>